The present invention generally relates to image forming apparatuses, and more particularly to an image forming apparatus provided with a fixing unit and/or an image forming unit suited for forming an image on a transported medium at a high speed.
Conventionally, a fixing unit using fixing rollers is used in an image forming apparatus in which a transport speed of a medium such as a recording sheet is relatively slow. When the medium transport speed is slow, an amount of heat per unit time to be supplied to the medium as the medium passes the fixing unit can be set small, and thus, a heat source of the fixing unit does not need to generate a large amount of heat.
Recently, however, there are demands to carry out a high-speed image formation in the image forming apparatus which uses the fixing unit, and the medium transport speed is increasing to meet such demands. For this reason, there are demands to increase the amount of heat per unit time that is supplied to the medium as the medium passes the fixing unit. In addition, there are also demands to suppress inconsistencies in a target temperature of the fixing unit during the medium transport to a minimum.
An image forming apparatus having a heat source with a large capacity has been proposed to meet the above described demands.
On the other hand, an image forming apparatus provided with a plurality of image forming apparatuses and capable of forming a color image has also been proposed.
However, as the thermal energy supplied by the heat source becomes large, there was a problem in that it is difficult to finely control the temperature of the fixing unit. In addition, at the time when the driving of the heat source is started, a large driving current is required to drive the heat source. For this reason, a power supply and the like of the image forming apparatus must be designed depending on the large driving current, thereby limiting the environment in which the image forming apparatus can be set up.
On the other hand, in the image forming apparatus provided with a plurality of image forming units, the same potential is applied with respect to all of the image forming units. Hence, when the medium is transported, the charge is absorbed by the medium and causes the potential to decrease, thereby introducing a problem in that the image transfer efficiency is poor.